Optimized PHY Frame Structure for OFDM Based Narrowband PLC

ABSTRACT

A method of operating a communication system is disclosed. The method includes forming a data frame having plural orthogonal frequency division multiplex (OFDM) symbols. A first set of preamble subcarriers is allocated to at least one of the OFDM symbols. A second set of data subcarriers is allocated to said at least one of the OFDM symbols.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.15/601,063, filed May 22, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/925,598, filed Oct. 28, 2015, now U.S. Pat. No.9,692,484, which claims priority to and the benefit of ProvisionalApplication No. 62/133,537, filed Mar. 16, 2015, each of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to power line communication(PLC) and, more particularly, to an optimized narrowband orthogonalfrequency division multiplex (OFDM) based physical (PHY) framestructure.

Powerline communications (PLC) include systems for communicating dataover the same medium that is used to transmit electric power toresidences, buildings, and other premises. Once deployed, PLC systemsmay enable a wide array of applications, including, for example,automatic meter reading and load control for utility-type applications,automotive uses such as charging electric cars, home automation forcontrolling appliances and lights, and computer networking for internetof things (IoT).

Various PLC standardizing efforts are currently being undertaken aroundthe world, each with its own unique characteristics. Examples ofcompeting PLC standards include the IEEE 1901.2, HomePlug AV, and ITU-TG.hn (e.g., G.9960 and G.9961) specifications. Generally speaking, PLCsystems may be implemented differently depending upon local regulationsand characteristics of local power grids. For example, the U.S. FCCimplementation of IEEE 1901.2 uses OFDM subcarriers from 10 kHz to 490kHz. CENELEC, the European standard, has various implementations usingOFDM subcarriers from 3 kHz to 148.5 kHz. ARIB, the Japanese standard,uses OFDM subcarriers from 10 kHz to 450 kHz. Another standardizationeffort includes, for example, the Powerline-Related Intelligent MeteringEvolution (PRIME) standard designed for OFDM-based (OrthogonalFrequency-Division Multiplexing) communications. The current or existingPRIME standard is the Draft Standard prepared by the PRIME AllianceTechnical Working Group (PRIME R1.3E) and earlier versions thereof.

Current and next generation narrowband PLC standards are directed tomulti-carrier based systems, such as orthogonal frequency divisionmultiplexing (OFDM) in order to get higher network throughput. OFDM usesmultiple orthogonal subcarriers to transmit data over frequencyselective channels. A conventional OFDM structure for a data frameincludes a preamble, followed by a physical layer (PHY) header, a mediaaccess control (MAC) header, followed by a data payload. However, PLCchannels are highly challenging environments for digital communicationbecause they suffer from periodic bursts of impulse noise, and thechannel impulse response also varies over time.

A conventional synchronization preamble structure for a narrowband OFDMPLC standard, such as IEEE 1901.2 (G3), includes 8 SYNCP symbolsfollowed by 1.5 SYNCM symbols. The synchronization symbols are typicallytransmitted at a higher (3 dB) rms voltage than the data payload, andthere is no cyclic prefix between adjacent symbols. Each SYNCP symbol isa known preamble sequence of different subcarriers phase shifted by amultiple of π/8. Subcarriers of the SYNCM symbol are phase shifted by πwith respect to SYNCP so that SYNCM=−SYNCP. For example, a SYNCP symbolmay be a chirp-like sequence of a specific binary sequence of 1 s and −1s or a constant amplitude, zero autocorrelation (CAZAC) sequence. Thedefinition of the SYNCP symbol for the FCC band in IEEE P1901.2 isdefined in section 6.6 for specific subcarriers or tones.

The preamble serves several purposes including: 1) indicating to othernodes in the PLC network that a transmission is in progress; 2)determining the frame boundary between the preamble and the PHY header,and between the PHY header and the data payload; 3) determining accuratechannel estimates; and 4) for frequency offset compensation. SYNCMsymbols help determine the frame boundary and indicate the end of thepreamble sequence. The repetitive SYNCP symbols also assist in preambledetection as receiver nodes are looking for the repetitive sequence ofsymbols in the PLC channel to determine whether or not a frame is on thepowerline. Multiple SYNCP symbols also help in obtaining more accuratechannel estimates by averaging the channel estimates across multiplesymbols to reduce noise. Improved channel estimates also help inimproving the header decoding performance when the header is coherentlymodulated with respect to the SYNCP preamble.

While preceding approaches provide improvement and standardization inPLC operation, the present inventors recognize that still furtherimprovements are possible. This is particularly true for high data ratePLC applications. Accordingly, the preferred embodiments described beloware directed toward this as well as improving upon the prior art.

BRIEF SUMMARY OF THE INVENTION

In a first embodiment of the present invention, there is disclosed amethod of operating a communication system. The method includes forminga data frame having plural orthogonal frequency division multiplex(OFDM) symbols. A first set of preamble subcarriers is allocated to atleast one of the OFDM symbols. A second set of data subcarriers isallocated to said at least one of the OFDM symbols.

In a second embodiment of the present invention, there is disclosed amethod of operating a communication system. The method includesreceiving a data frame having plural orthogonal frequency divisionmultiplex (OFDM) symbols. A first set of preamble signals is receivedfrom at least one of the OFDM symbols. A second set of data signals isreceived from said at least one of the OFDM symbols.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagram of a power line communication (PLC) environment ofthe present invention;

FIG. 2 is a block diagram of an IEEE 1901.2 (G3) compatible device ofthe present invention;

FIG. 3 is an IEEE 1901.2 (G3) compatible coherent frame structure of thepresent invention;

FIG. 4 is a diagram of a circuit for preamble symbol generationaccording to a first embodiment of the present invention;

FIG. 5A is a diagram of sequential preamble symbols as generated by thecircuit of FIG. 4, for a first logic level of a modulation controlscheme (MCS) signal;

FIG. 5B is a diagram of sequential preamble symbols as generated by thecircuit of FIG. 4, for a second logic level of the modulation controlscheme (MCS) signal;

FIG. 6 is a diagram of a circuit for preamble symbol generationaccording to a second embodiment of the present invention;

FIG. 7 is a diagram of sequential preamble symbols as generated by thecircuit of FIG. 6, for respective odd and even preamble symbols; and

FIG. 8 is a diagram of throughput gain as a function of payload sizeaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is an electric power distribution system isdepicted according to the present invention. Medium voltage (MV) powerlines 103 from substation 101 typically carry voltage in the tens ofkilovolts range. Transformer 104 steps the MV power down to low voltage(LV) power on LV lines 105, carrying voltage in the range of 100-240VAC. Transformer 104 is typically designed to operate at very lowfrequencies in the range of 50-60 Hz. Transformer 104 does not typicallyallow high frequencies, such as signals greater than 100 kHz, to passbetween LV lines 105 and MV lines 103. LV lines 105 feed power tocustomers via meters 106 a-n, which are typically mounted on the outsideof residences 102 a-n. Although referred to as residences, premises 102a-n may include any type of building, facility or location whereelectric power is received and/or consumed. A breaker panel, such aspanel 107, provides an interface between meter 106 n and electricalwires 108 within residence 102 n. Electrical wires 108 deliver power tooutlets 110, switches 111, and other electric devices within residence102 n.

The power line topology illustrated in FIG. 1 may be used to deliverhigh-speed communications to residences 102 a-n. In someimplementations, power line communication (PLC) modems or gateways 112a-n may be coupled to LV power lines 105 at meter 106 a-n. PLC gateways112 a-n may be used to transmit and receive data signals over MV/LVlines 103/105. Such data signals may be used to support metering andpower delivery applications, communication systems, high speed internet,telephony, video conferencing, and video delivery, to name a few. Bytransporting telecommunications data signals over a power transmissionnetwork, there is no need to install new cabling to each subscriber 102a-n. Thus, by using existing electrical distribution systems to carrydata signals, significant cost savings are possible.

PLC modems or gateways 112 a-n at residences 102 a-n use the MV/LV powergrid to carry data signals to and from PLC data concentrator 114 withoutrequiring additional wiring. Concentrator 114 may be coupled to eitherMV line 103 or LV line 105. Modems or gateways 112 a-n may supportapplications such as high-speed broadband internet links, narrowbandcontrol applications, and low bandwidth data collection applications, orthe like. In a home environment, for example, modems or gateways 112 a-nmay further enable home and building automation in heat and airconditioning, lighting, and security. Also, PLC modems or gateways 112a-n may enable AC or DC charging of electric vehicles and otherappliances. An example of an AC or DC charger is illustrated as PLCdevice 113. Outside the premises, power line communication networks mayprovide street lighting control and remote power meter data collection.

One or more data concentrators 114 may be coupled to control center 130,which may be a utility company, via network 120. Network 120 mayinclude, for example, an internet protocol (IP) based network, acellular network, a WiFi network, a WiMax network, or the like. As such,control center 130 may be configured to collect power consumptioninformation and other types of relevant information from gateways 112and devices 113 through concentrator 114. Additionally, control center130 may be configured to implement smart grid policies and otherregulatory or commercial rules by communicating such rules to eachgateway 112 or device 113 through concentrator 114.

In some embodiments, concentrator 114 may be a base node for a PLCdomain, each such domain comprising downstream PLC devices thatcommunicate with control center 130 through a respective concentrator114. For example, in FIG. 1, devices 106 a-n, 112 a-n, and 113 may allbe considered part of the PLC domain that has data concentrator 114 asits base node. In other scenarios other devices may be used as the basenode of a PLC domain. In a typical situation, multiple nodes may bedeployed in a given PLC network, and at least a subset of those nodesmay be tied to a common clock through a backbone such as Ethernet ordigital subscriber loop (DSL).

Still referring to FIG. 1, meter 106, gateways 112, PLC device 113, anddata concentrator 114 may each be coupled to or otherwise include a PLCmodem or the like. The PLC modem may include transmitter and receivercircuitry to facilitate the device's connection to power lines 103, 105,and/or 108.

FIG. 2 is a block diagram of an exemplary low cost, low power IEEE1901.2 compatible device 200 that may be used in blocks 112 a-n (FIG. 1)according to the present invention. The diagram illustrates an OFDMtransmitter 210 and receiver 220 for use in a power line communicationnode for PLC over a power line 202. As discussed above, the power linechannel is very hostile. Channel characteristics and parameters varywith frequency, location, time and the type of equipment connected toit. The lower frequency regions from 10 kHz to 200 kHz used in G3 PLCand in IEEE 1901.2 are especially susceptible to interference.Furthermore, the power line is a very frequency selective channel.Besides background noise, it is subject to impulsive noise oftenoccurring at 50/60 Hz, and narrowband interference and group delays upto several hundred microseconds.

Preamble circuit 232 produces a preamble to synchronize each transmitteddata frame with a receiving device. Preamble sequence allocator circuit230 determines which tones in an OFDM symbol will be occupied by thepreamble. Data 211 and a frame control header (FCH) 212 are provided byan application via a media access layer (MAC) of the communicationprotocol. Similar to the preamble sequence allocator circuit 230, datasequence allocator circuit 234 determines which tones in an OFDM symbolwill be used by data circuit 211 or FCH circuit 212. An OFDM signal isgenerated by performing an inverse fast Fourier transform (IFFT) 215 onthe complex valued signal points that are produced by differentiallyencoded phase modulation from forward error correction encoder 213 usingReed Solomon encoding. Tone mapping 214 is performed to allocate thesignal points to individual subcarriers. An OFDM symbol is built byappending a cyclic prefix (CP) 216 to the beginning of each blockgenerated by IFFT 215. The length of a cyclic prefix is chosen so that achannel group delay will not cause successive OFDM Symbols or adjacentsub-carriers to interfere. The OFDM symbols are then windowed 217 andimpressed on power line 202 via analog front end (AFE) 218. AFE 218provides isolation of transmitter 210 from the 50/60 Hz power linevoltage.

Similarly, receiver 220 receives OFDM signals from power line 202 viaAFE 221 that isolates receiver 220 from the 50/60 HZ power line voltage.OFDM demodulator 222 removes the CP, converts the OFDM signal to thefrequency domain using a fast Fourier transform (FFT), and performsdemodulation of the differential binary or quadrature phase shift keyed(DBPSK, DQPSK) symbols. FEC decoder 223 performs error correction usingReed Solomon decoding and then descrambles the symbols to producereceived data 224. Frame control header 225 information is also producedby FEC decoder 220, as defined by the G3 and IEEE 1901.2 PLC standards.Similar to the transmitter 210, receiver 220 also has a preamblesequence allocator circuit 236 and data sequence allocator circuit 238to indicate which tones are used for the preamble and which tones areused for data in any OFDM symbol.

A blind channel estimation technique may be used for link adaptation.Based on the quality of the received signal, the receiver decides on themodulation scheme to be used, as defined in the PLC standards. Moreover,the system may differentiate the subcarriers with a bad signal to noiseratio (SNR) and not transmit data on them.

Transmitter 210 and receiver 220 may be implemented using a digitalsignal processor (DSP) or another type of microprocessor that isexecutes control software instructions stored in memory. For example,the processor may perform operations such as FEC encoding, mapping andOFDM modulation, demodulation and FEC decoding in software. In otherembodiments, portions or all of the transmitter or receiver may beimplemented with hardwired control logic. The analog front ends 218 and221 require analog logic and isolation transformers that can withstandthe voltage levels present on the power line.

A G3 and IEEE 1901.2 PLC system is specified to have the ability tocommunicate in both low voltage (LV) power lines, typically 100-240 VAC,as well as medium voltage (MV) power lines up to approximately 12 kV bycrossing LV/MV transformers. This means that the receiver on the LV sidemust be able to detect the transmitted signal after it has been severelyattenuated as a result of going through a MV/LV transformer. As thesignal goes through the transformer it is expected to experience overallsevere attenuation in its power level as well as frequency-dependentattenuation that attenuates higher frequencies. Both transmitter andreceiver have mechanisms to compensate for this attenuation. Thetransmitter has the capability to adjust its overall signal level aswell as shape its power spectrum based on tone map information providedby a target receiver, while the receiver has both an analog and digitalautomatic gain control (AGC) in order to achieve enough gain tocompensate for the overall attenuation.

Turning now to FIG. 3, there is a coherent frame structure forcommunication between the network of FIG. 1 and the device of FIG. 2according to the present invention. The frame includes a preamble, aframe control header (FCH) and payload data. The preamble includessynchronization symbols such as SYNCP 300 and SYNCM 302 and isterminated by a half SYNCM symbol 304, which is preferably a repetitionof the first half of SYNCM 302. The preamble is separated from the FCHby an overlap region 306. The exemplary FCH includes 12 OFDM symbols,but this may vary with different band plans. The FCH is coherentlymodulated and contains information regarding the current frame such asthe type of frame, the tone map index, and the length of the frame. EachFCH symbol is preceded by a respective guard interval (GI). For example,the FCH1 310 is preceded by respective GI 308, and each FCH symbol isseparated by an overlap region. Symbols S1 and S2 are inserted betweenthe FCH and the payload data (DATA). Symbol S2 is similar to SYNCP 300except that it includes a cyclic prefix, GI, and overlap regions. SymbolS1 is an inverted version of S2 (−S2). The payload data follows symbolsS1 and S1 and includes data in respective OFDM symbols. The firstsymbol, for example, includes overlap 312, GI 314, and data 316.

Referring next to FIG. 4, there is a diagram of a circuit for preamblesymbol generation according to a first embodiment of the presentinvention. The circuit includes IFFT and parallel-to-serial (P/S)converter 215 from FIG. 2. Cyclic prefix circuit 216 appends a cyclicprefix from the end of the P/S converter output to the beginning of theserial output to complete the OFDM symbol. As previously discussed, IEEE1901.2 has specified 128 tones for FCC narrowband PLC. This requires aminimum of N=256 IFFT samples. In this exemplary embodiment, however,only K=72 of these tones are used, and the remaining 56 tones at theends of the IFFT are unused and set to zero. Preamble tones 1-36(P₁-P_(K/2)) are applied as inputs to the IFFT circuit. Multiplexcircuit 400 is coupled to receive preamble tones 37-72 (P_(K/2)-P_(K))and 36 data tones (X₁-X_(K/2)). Multiplex circuit 400 selectivelyapplies the 36 preamble tones or the 36 data tones to the IFFT circuitin response to a modulation control scheme (MCS) signal.

Operation of the circuit of FIG. 4 will now be described with referenceto FIGS. 5A-5B. When the PLC signal-to-noise ratio (SNR) is good, a highdata rate MCS may be selected for the frame of FIG. 3. In this case, 36preamble tones are sufficient for synchronization detection at areceiver (FIG. 2). Thus, MCS has a high logic level and multiplexcircuit 400 applies 36 data tones to the IFFT circuit. Sequential SYNCPsymbols, therefore, have a structure as illustrated in FIG. 5A, wherethe vertical axis is subcarrier frequency and the horizontal axis istime. Each SYNCP OFDM symbol includes 36 preamble tones 500 and 36 datatones 502. The preamble tones are preferably contiguous within the OFDMsymbol to provide a better correlation profile. This greatly increasesdata throughput when the PLC SNR is good. When the PLC signal-to-noiseratio (SNR) is compromised by noise, a lower data rate MCS may beselected for the frame of FIG. 3. In this case, 72 preamble tones may berequired for synchronization detection. Thus, MCS has a low logic leveland multiplex circuit 400 applies 36 additional preamble tones(P_(K/2)-P_(K)) to the IFFT circuit. Sequential SYNCP symbols,therefore, have a structure as illustrated in FIG. 5B. Each SYNCP OFDMsymbol includes 72 preamble tones 504 and no data tones. In a first modeof operation, a receiver operates on a static allocation of data foreach OFDM preamble symbol. This is preferably a default mode. In asecond mode of operation, the receiver operates on a semi-persistent oradaptive allocation as determined by a received data frame. This mode ispreferably adapted to the communication system SNR. This embodiment ofthe present invention advantageously increases data throughput when thePLC SNR is good and reverts to normal data throughput in a high noiseenvironment.

Referring now to FIG. 6, there is a diagram of a circuit for preamblesymbol generation according to a second embodiment of the presentinvention. The circuit includes IFFT and parallel-to-serial (P/S)converter 215 and cyclic prefix circuit 216. Multiplex circuit 600selectively applies the 36 preamble tones (P₁-P_(K/2)) for odd-numberedsymbols or 36 data tones (X₁-X_(K/2)) for even-numbered symbols inresponse to control signal EVEN. Likewise, multiplex circuit 602selectively applies the 36 data tones for odd-numbered symbols or 36preamble tones for even-numbered symbols in response to control signalEVEN.

Operation of the circuit of FIG. 6 will now be described with referenceto FIG. 7. For odd-numbered symbol 700 (Symbol 1), control signal EVENhas a low logic state. Thus, multiplex circuit 600 applies preambletones P₁-P_(K/2) to IFFT circuit 215, and multiplex circuit 602 appliesdata tones X₁-X_(K/2) to IFFT circuit 215. Symbol 700, therefore,includes 36 preamble tones 702 at the upper frequency range and 36 datatones 704 at the lower frequency range. For even-numbered symbol 706(Symbol 2), control signal EVEN has a high logic state. Thus, multiplexcircuit 600 applies data tones X₁-X_(K/2) to IFFT circuit 215, andmultiplex circuit 602 applies preamble tones P₁-P_(K/2) to IFFT circuit215. Symbol 706, therefore, includes 36 data tones 708 at the upperfrequency range and 36 preamble tones 710 at the lower frequency range.In a first mode of operation, a receiver operates on a static allocationof alternating preamble and data subcarriers in each OFDM preamblesymbol. This is preferably a default mode. In a second mode ofoperation, the receiver operates on a semi-persistent or adaptiveallocation as determined by a received data frame. This mode ispreferably adapted to the communication system SNR and desiredthroughput. This embodiment of the present invention advantageouslyimproves frequency diversity gain by alternating preamble and datafrequencies in adjacent OFDM symbols.

Referring to FIG. 8, there is a diagram of throughput gain as a functionof payload size according to the present invention. In the foregoingembodiment of the present invention, throughput gain increases from 19%to 34% as payload data increases from 30 to 70 bytes. Further increasesin payload data size are accompanied by a gradual decrease in throughputgain. This is because additional data in the preamble becomes lesssignificant as payload data (after FCH) in the frame increases. At 480bytes of payload data, the throughput gain decreases to 14%.

Embodiments of the present invention may be readily adapted to otherframe structures as one of ordinary skill in the art having access tothe instant specification will understand. For other applications, thenumber of used preamble symbol tones K may be more or less than 72.Likewise, the IFFT samples may be more or less than 256. Althoughprevious embodiments have specifically addressed addition of payloaddata to SYNCP symbols, payload data may also be added to SYNCM symbolsor to frame symbols S1 and S2 of the FCH. Moreover, embodiments of FIGS.4 and 6 may be combined to selectively include or omit data tones inresponse to a modulation control scheme, and the included data tones mayalternate with preamble tones in alternating OFDM symbols.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling with the inventive scope as defined by the followingclaims. Furthermore, embodiments of the present invention may beimplemented in software, hardware, or a combination of both. Othercombinations will be readily apparent to one of ordinary skill in theart having access to the instant specification.

What is claimed is:
 1. A method comprising: selecting a high data ratemodulation and coding scheme for communication in response to asignal-to-noise ratio of a communication line; based on selecting thehigh data rate modulation and coding scheme, generating a first framethat includes a first preamble and a first data portion; transmitting,using the first frame, the first preamble and the first data portionconcurrently; selecting a low data rate modulation and coding scheme forcommunication in response to the signal-to-noise ratio of thecommunication line; based on selecting the low data rate modulation andcoding scheme, generating a second frame that includes a secondpreamble; and transmitting, using the second frame, the second preamblewithout concurrently transmitting a second data portion.
 2. The methodof claim 1, wherein: the generating of the second frame allocates a setof N tones for transmitting the second preamble; and the generating ofthe first frame allocates the set of N tones for transmitting the firstpreamble and the first data portion.
 3. The method of claim 2, whereinthe set of N tones are contiguous orthogonal frequency divisionmultiplex (OFDM) symbol tones.
 4. The method of claim 2, wherein thegenerating of the first frame allocates a first set of N/2 tones fortransmitting the first preamble and a second set of N/2 tones fortransmitting the first data portion.
 5. The method of claim 4 furthercomprising generating a third frame that includes a third preamble and athird data portion, wherein the generating of the third frame allocatesthe first set of N/2 tones for transmitting the third data portion andthe second set of N/2 tones for transmitting the third preamble.
 6. Themethod of claim 4 wherein tones of the first set of N/2 tones are higherin frequency than tones of the second set of N/2 tones.
 7. The method ofclaim 1, wherein the communication line includes a power line.
 8. Themethod of claim 1 further comprising: generating, by an inverse fastFourier transform (IFFT), a first orthogonal frequency divisionmultiplex (OFDM) symbol for the first frame; transmitting the first OFDMsymbol over the communication line; generating, by the IFFT, a secondOFDM symbol for the second frame; and transmitting the second OFDMsymbol over the communication line.
 9. The method of claim 8 furthercomprising: serializing the first OFDM symbol prior to the transmittingof the first OFDM symbol; and serializing the second OFDM symbol priorto the transmitting of the second OFDM symbol.
 10. A communicationdevice comprising: at least one processor; a non-transitory computerreadable storage medium storing instructions for execution by the atleast one processor; wherein the instructions cause the at least oneprocessor to: select a first data rate for communication in response toa signal-to-noise ratio of a communication line being above a threshold;based on selecting the first data rate, generate a first frame thatincludes a first preamble and a first data portion; transmit, using thefirst frame, the first preamble and the first data portion concurrently;select a second data rate for communication in response to thesignal-to-noise ratio of the communication line being below thethreshold; based on selecting the second data rate, generate a secondframe that includes a second preamble without a second data portion; andtransmitting, using the second frame, the second preamble.
 11. Thecommunication device of claim 10, wherein: the instructions to generatethe first frame include instructions to allocate a set of orthogonalfrequency division multiplex (OFDM) symbol tones for transmitting thefirst preamble and the first data portion; and the instructions togenerate the second frame include instructions to allocate the set ofOFDM symbol tones for transmitting the second preamble.
 12. Thecommunication device of claim 11, wherein the instructions to allocatethe set of OFDM symbol tones for transmitting the first preamble and thefirst data portion allocate a first half of the set of OFDM symbol tonesfor transmitting the first preamble and a second half of the set of OFDMsymbol tones for transmitting the first data portion.
 13. Thecommunication device of claim 12, wherein the instructions includefurther instructions to, based on selecting the first data rate,generate a third frame that includes a third preamble and a third dataportion such that the first half of the set of OFDM symbol tones isallocated for transmitting the third data portion and the second half ofthe set of OFDM symbol tones is allocated for transmitting the thirdpreamble.
 14. The communication device of claim 10, wherein thecommunication line includes a power line.
 15. The communication deviceof claim 10, wherein: the instructions to transmit, using the firstframe, the first preamble and the first data portion includeinstructions to: generate, by an inverse fast Fourier transform (IFFT),a first orthogonal frequency division multiplex (OFDM) symbol for thefirst frame; and transmit the first OFDM symbol over the communicationline; and the instructions to transmit, using the second frame, thesecond preamble include instructions to: generate, by the IFFT, a secondOFDM symbol for the second frame; and transmit the second OFDM symbolover the communication line.
 16. A method comprising: selecting a firstdata rate for communication in response to a first signal-to-noise ratioof a first communication line exceeding a threshold; based on selectingthe first data rate, generating a first frame that includes a firstpreamble and a first data portion; transmitting, using the first frame,the first preamble and the first data portion; selecting a second datarate for communication in response to a second signal-to-noise ratio ofa second communication line being below the threshold; based onselecting the second data rate, generating a second frame that includesa second preamble; and transmitting, using the second frame, the secondpreamble without transmitting a second data portion.
 17. The method ofclaim 16, wherein: the generating of the second frame includesallocating a set of orthogonal frequency division multiplex (OFDM) tonesfor transmitting the second preamble; and the generating of the firstframe includes allocating the set of OFDM tones for transmitting thefirst preamble and the first data portion.
 18. The method of claim 17,wherein the allocating of the set of OFDM tones for transmitting thefirst preamble and the first data portion allocates a first half of theset of OFDM tones for transmitting the first preamble, and a second halfof the set of OFDM tones for transmitting the first data portion. 19.The method of claim 18 further comprising generating a third frame thatincludes a third preamble and a third data portion by allocating thefirst half of the set of OFDM tones for transmitting the third dataportion, and a second half of the set of OFDM tones for transmitting thethird preamble.
 20. The method of claim 17, wherein the set of OFDMtones is contiguous.